1. Field
The present invention relates to optical filters wherein the spatial and/or spectral distribution of radiation in a light beam is controlled actively or passively by the filter. In particular the invention is concerned with filters as described above which are constructed from thin films of polymeric material.
2. Prior Art
Numerous prior art filters have been constructed using thin layers of optically transparent material. The most popular have been filter structures wherein alternate layers have relatively high and low indices of refraction and the thickness of the layers is an integral number of quarter-wavelengths at the center frequency of the filter. The usual substrate material is glass or a crystalline substance upon which dielectric materials are deposited after evaporation in a vacuum chamber. Such filters are simple to analyze, but are obviously limited, mostly by the constraints imposed by their methods of manufacture.
Improvements in computer programming have now made it possible to design filters having any frequency characteristic without presuming either the discrete layer thicknesses or the discrete refractive indices of the materials used. In the final analysis these parameters are used as trade-offs to get the best possible filter characteristics within the constraints of available materials and processing techniques. One important conclusion that has emerged from these computational programs is that filters which have very narrow bandwidths can be made from materials which have only small differences in their indices of refraction. A computer program as described above entitled "OPTICAL THIN FILM SYNTHESIS PROGRAM BASED ON THE USE OF FOURIER TRANSFORMS" by J. A. Dobrowolski and D. Lowe was published in Applied Optics, Vol. 17, No. 19, 1 October 1978, pp 3039-3050.
Two additional advances in the optical art have had considerable impact on the design of thin layered filters. The first is the development of organic polymers as optical materials, and the second is the development of high-power laser sources, which produce extremely intense electric fields and have very narrow frequency bands, e.g., a few angstroms. The polymers may have centric or acentric lattice structures. These polymers interact non-linearly with externally applied electric fields and/or the intense electric fields associated with laser beams to produce marked changes in the indices of refraction. An object of the present invention is to utilize these phenomena in forming novel radiation-control devices based on layered structures of optical polymers.